The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.
It is known in the prior art that high molecular weight polymers can be prepared by copolymerizing carbon dioxide with epoxy compounds in order to provide the corresponding poly(alkylene carbonates). The polymers which are produced are typically alternating copolymers of the carbon dioxide and epoxide monomers. Because such high molecular weight polymers decompose relatively cleanly, they find use in foam moulding applications and as binders for ceramic or metallic particles in sintered moulding procedures. The polymers can also be fabricated into films and other shaped articles and used in blends with other polymers for various applications such as adhesives.
Rokicki and Kuran, “The Application of Carbon Dioxide as a Direct Material for Polymer Synthesis in Polymerization and Polycondensation Reactions” J. Macromol. Sci.-Rev. Macromol. Chem., C21(1), 135-136 (1981), present a survey of scientific literature on the use of carbon dioxide in polymerization and polycondensation reactions and describe, inter alia, the copolymerization of carbon dioxide with oxiranes using organozinc catalysts, such as diethylzinc-pyrogallol and zinc carboxylates as well as metallo-organic catalysts of cobalt, chromium, nickel, magnesium and aluminium, thereby indicating that a relatively large number of catalysts are active in promoting the copolymerization between carbon dioxide and oxiranes. Catalysts based on diethylzinc predominate in reports on the alternate copolymerization of carbon dioxide and oxiranes, but coordination catalysts, for example, metal carboxylates, have been less widely studied. Zinc derivatives, however, are said to exhibit higher activity than derivatives of cobalt or cadmium, while derivatives of aluminium, magnesium, chromium and nickel lead generally to low molecular weight polymers.
Soga, et al., Polymer Journal, 13, pages 407-410 (1981), disclose alternating copolymerization of carbon dioxide and propylene oxide with catalysts prepared from zinc hydroxide and dicarboxylic acids.
Soga, Nippon Kagakkaishi, Vol. 2, 295-300 (1982), investigates several types of catalyst systems which promote alternate copolymerization of carbon dioxide and alkylene oxides as possible improvements over the known catalyst system of diethylzinc and water. These alternate catalysts include metal oxide-supported diethylzinc, acetic acid salts of cobalt and zinc, reaction products of zinc hydroxide and dicarboxylic acids and metal oxide-supported zinc, cobalt and aluminium halides. The zinc dicarboxylate formed by reacting zinc hydroxide with glutaric acid was said to be about 3 times as active as the diethyl zinc.H2O system, but the catalyst prepared by reacting zinc oxide with glutaric acid was said to offer no improvement at all over the diethyl zinc.H2O system.
These prior art methods to synthetically prepare zinc glutarate (ZnGA) comprise mixing zinc oxide with glutaric acid in an organic solvent such as toluene or benzene at a certain temperature and for a certain time period. The glutaric acid dissolves in the organic solvent and reacts with ZnO suspended particles in solution to produce the catalyst (e.g. see Wang, S. J., et al., Synthesis and Characterization of Alternating Copolymer from Carbon Dioxide and Propylene Oxide. Journal of Applied Polymer Science, 2002. 85: p. 2327-2334). However, the major drawback in the preparation of ZnGA catalyst is the use of toxic, carcinogenic and flammable organic solvents like toluene or benzene.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.